Mitochondria are organelles within cells that are largely responsible for converting metabolic fuel into a form of energy that can be used by the rest of the cell. The long term objective of this research is to understand the role of mitochondrial function/dysfunction may play in the etiology of metabolic disease. The overriding hypothesis of this project is that the nutritional imbalance in skeletal muscle created by an oversupply of metabolic substrates (over nutrition, particularly from high fat diets) coupled with low energy demand (sedentary lifestyle) increases the propensity for mitochondrial hydrogen peroxide generation and emission, representing the potential primary factor for the decrease in insulin sensitivity associated with diet-induced obesity. Using a novel approach to study mitochondrial function in human myofibers in situ, the Specific Aims of this project are: 1) to determine the impact of obesity on the regulation of mitochondrial function, cellular redox balance and insulin sensitivity in skeletal muscle of sedentary lean verses obese young adults; 2) to determine how high calorie/high fat intake in lean humans acutely and/or chronically affects mitochondrial function, cellular redox balance and insulin sensitivity; 3) to determine whether increased physical activity restores redox balance and insulin sensitivity in obese individuals or in lean individuals consuming a high calorie/high fat diet; and 4) to investigate in cultured human primary myotubes whether mitochondrial ROS emission is a primary cause of lipid-induced insulin resistance.